Semiconductor device including side surface conductor contact

ABSTRACT

The memory capacity of a DRAM is enhanced. A semiconductor memory device includes a driver circuit including part of a single crystal semiconductor substrate, a multilayer wiring layer provided over the driver circuit, and a memory cell array layer provided over the multilayer wiring layer. That is, the memory cell array overlaps with the driver circuit. Accordingly, the integration degree of the semiconductor memory device can be increased as compared to the case where a driver circuit and a memory cell array are provided in the same plane of a substrate containing a singe crystal semiconductor material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/292,667, filed Mar. 5, 2019, now allowed, which is a continuation ofU.S. application Ser. No. 15/701,499, filed Sep. 12, 2017, now U.S. Pat.No. 10,249,626, which is a continuation of U.S. application Ser. No.15/148,000, filed May 6, 2016, now U.S. Pat. No. 9,786,668, which is acontinuation of U.S. application Ser. No. 14/336,118, filed Jul. 21,2014, now U.S. Pat. No. 9,337,345, which is a continuation of U.S.application Ser. No. 13/344,921, filed Jan. 6, 2012, now U.S. Pat. No.8,811,064, which claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2011-005401 on Jan. 14, 2011, all of whichare incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to semiconductor memory devices. Inparticular, one embodiment of the present invention relates to asemiconductor memory device including a memory cell including atransistor whose channel region is formed of an oxide semiconductor.

2. Description of the Related Art

In recent years, a metal oxide having semiconductor characteristics,which is called an oxide semiconductor exhibiting a high mobility anduniform element characteristics, has attracted attention as a materialof a transistor. Metal oxides are used for a variety of applications.For example, indium oxide is used as a material of a pixel electrode ina liquid crystal display device. Examples of such a metal oxide havingsemiconductor characteristics include tungsten oxide, tin oxide, indiumoxide, and zinc oxide; transistors which include such a metal oxidehaving semiconductor characteristics in their channel regions are known(Patent Documents 1 and 2).

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2007-123861-   Patent Document 2: Japanese Published Patent Application No.    2007-096055

SUMMARY OF THE INVENTION

As semiconductor memory devices, there are volatile memories such as aDRAM and an SRAM and nonvolatile memories such as a mask ROM, an EPROM,an EEPROM, a flash memory, and a ferroelectric memory, most of which hasalready been put into practical use with the use of a single crystalsemiconductor substrate. Among the above-described memory devices, aDRAM has a simple structure in which a memory cell consists of atransistor and a capacitor, and the number of semiconductor elements inthe memory cell of the DRAM is fewer than that of semiconductor elementsin other memory devices such as an SRAM. Therefore, the memory capacityper unit area can be increased as compared to other memory devices,resulting in lower cost.

A DRAM is suitable for large memory capacity as described above,nonetheless, it is necessary to increase the memory capacity per unitarea as in other memory devices in order to improve the integrationdegree of an integrated circuit while suppressing an increase in chipsize. Therefore, it is necessary to reduce the area of a capacitor forholding charge in each memory cell to reduce the area of each memorycell.

Specifically, for a reduction in the area of each memory cell, atechnique of providing a capacitor in a deep groove formed in asemiconductor substrate (the capacitor is called a trench capacitor), atechnique of providing a long capacitor directly above or substantiallydirectly above a semiconductor substrate (the capacitor is called astack capacitor), and the like have been developed. Specifically,capacitors with aspect ratios of higher than or equal to 50 have beendeveloped. Further, a technique of providing a plurality of layeredwiring layers over such a semiconductor substrate to electricallyconnect a significant amount of highly integrated semiconductor elementsprovided for the semiconductor substrate (the technique is called amultilayer wiring technique), and the like have been developed.

One object of one embodiment of the present invention is to enhance thememory capacity of a DRAM.

In a semiconductor memory device according to one embodiment of thepresent invention, a memory cell array is provided over a driver circuitincluding part of a substrate containing a single crystal semiconductormaterial with a multilayer wiring layer provided therebetween.

Specifically, one embodiment of the present invention is a semiconductormemory device including: a driver circuit including part of a singlecrystal semiconductor substrate; a multilayer wiring layer which isprovided over the driver circuit and includes a plurality of wiringsformed of copper or a copper alloy; and a memory cell array layer whichis provided over the multilayer wiring layer and includes a plurality ofmemory cells arranged in a matrix manner. The plurality of memory cellsare electrically connected to the driver circuit through respective atleast ones of the plurality of wirings. Each memory cell includes atransistor whose channel region is formed of an oxide semiconductor anda capacitor whose one electrode is electrically connected to one of asource and a drain of the transistor.

In a semiconductor memory device according to one embodiment of thepresent invention, a memory cell array can be provided to overlap with adriver circuit including part of a substrate containing a single crystalsemiconductor material. Accordingly, the integration degree of thesemiconductor memory device can be increased as compared to the casewhere a driver circuit and a memory cell array are provided in the sameplane of a substrate containing a single crystal semiconductor material.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates a structure example of a semiconductor memory device;

FIG. 2 illustrates a structure example of a transistor in a drivercircuit;

FIGS. 3A to 3H illustrate an example of a method for manufacturing atransistor;

FIG. 4 illustrates a structure example of a wiring layer;

FIGS. 5A to 5H illustrate an example of a method for manufacturing awiring layer;

FIG. 6 illustrates a structure example of a memory cell;

FIGS. 7A to 7H illustrate an example of a method for manufacturing atransistor in a memory cell;

FIG. 8 is a block diagram illustrating a structure example of amicroprocessor; and

FIGS. 9A to 9C illustrate concrete examples of a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are hereinafter described withreference to the accompanying drawings. The present invention is notlimited to the description below, and it is easily understood for thoseskilled in the art that a variety of modifications can be made withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention is not construed as being limited to the contentof the embodiments described hereinafter.

First, an example of a structure of a semiconductor memory deviceaccording to one embodiment of the present invention is described usingFIGS. 1, 2, 3A to 3H, 4, 5A to 5H, 6, and 7A to 7H.

<Structure Example of Semiconductor Memory Device>

FIG. 1 illustrates an example of a structure of a semiconductor memorydevice. The semiconductor memory device shown in FIG. 1 includes adriver circuit 100 including part of a substrate 10 containing a singlecrystal semiconductor material, a multilayer wiring layer 20 providedover the driver circuit 100, and a memory cell array layer 30 which isprovided over the multilayer wiring layer 20 and includes a plurality ofmemory cells 300 arranged in a matrix manner.

The driver circuit 100 consists of a plurality of semiconductor elementsusing the substrate 10 containing a single crystal semiconductormaterial. With the driver circuit 100, data is written and readinto/from each memory cell 300.

The multilayer wiring layer 20 includes a plurality of wiring layers 20a and 20 b each having a plane with a plurality of wirings 200 provided.With the multilayer wiring layer 20, the semiconductor elements in thedriver circuit 100 are electrically connected to each other, and thedriver circuit 100 is electrically connected to each of the plurality ofmemory cells 300. The planes with the plurality of wirings 200 providedare layered in the multilayer wiring layer 20. Specifically, a pluralityof insulating layers in each of which the plurality of wirings 200 areembedded are stacked. The plurality of wirings 200 in one plane areelectrically connected to the plurality of wirings 200 in another planethrough a contact plug 201 formed in the insulating layer. Although themultilayer wiring layer 20 consists of the two wiring layers 20 a and 20b in FIG. 1, the multilayer wiring layer 20 according to one embodimentof the present invention is not limited to two layers: the multilayerwiring layer 20 may consist of three or more layers. Further, a bit lineof the memory cell may be formed of one or more layers of the multilayerwiring layer 20.

Each memory cell 300 includes a transistor 301 and a capacitor 302 whoseone electrode is electrically connected to one of a source and a drainof the transistor 301.

<Structure Example of Driver Circuit 100>

Hereinafter, an example of a transistor formed using the substrate 10containing a single crystal semiconductor material in the driver circuit100 is described using FIG. 2.

A transistor 160 illustrated in FIG. 2 includes a channel region 116provided in the substrate 10, a pair of impurity regions 114 a and 114 band a pair of high concentration impurity regions 120 a and 120 b (theseregions are also collectively referred to simply as impurity regions)each pair of which is provided with the channel region 116 providedtherebetween, a gate insulating film 108 provided over the channelregion 116, a gate electrode 110 provided over the gate insulating film108, a source electrode 130 a which is electrically connected to theimpurity region 114 a, and a drain electrode 130 b which is electricallyconnected to the impurity region 114 b.

A sidewall insulating layer 118 is provided on a side surface of thegate electrode 110. The substrate 10 includes the pair of highconcentration impurity regions 120 a and 120 b in regions which do notoverlap with the sidewall insulating layer 118, and includes a pair ofmetal compound regions 124 a and 124 b over the pair of highconcentration impurity regions 120 a and 120 b. Further, an elementisolation insulating layer 106 is provided over the substrate 10 so asto surround the transistor 160. An interlayer insulating layer 126 andan interlayer insulating layer 128 are provided so as to cover thetransistor 160. The source electrode 130 a is electrically connected tothe metal compound region 124 a through an opening formed in theinterlayer insulating layers 126 and 128, and the drain electrode 130 bis electrically connected to the metal compound region 124 b through anopening formed in the interlayer insulating layers 126 and 128. In otherwords, the source electrode 130 a is electrically connected to the highconcentration impurity region 120 a and the impurity region 114 athrough the metal compound region 124 a, and the drain electrode 130 bis electrically connected to the high concentration impurity region 120b and the impurity region 114 b through the metal compound region 124 b.

<Example of Manufacturing Method of Transistor>

Next, an example of a method for manufacturing the transistor 160 isdescribed using FIGS. 3A to 3H. The transistor 160 can be formed notonly by the following method but also any known method.

First, the substrate 10 containing a single crystal semiconductormaterial is prepared (see FIG. 3A). As the substrate 10, a singlecrystal semiconductor substrate of silicon, silicon carbide, silicongermanium, or gallium arsenide, an SOI substrate in which a singlecrystal layer thereof is provided on an insulating layer, or the likecan be used. Note that although in general, the “SOI substrate” means asubstrate in which a silicon semiconductor layer is provided on aninsulating surface, the “SOI substrate” in this specification and thelike also includes in its category a substrate in which a semiconductorlayer containing a material other than silicon is provided on aninsulating surface. That is, the semiconductor layer included in the“SOT substrate” is not limited to a silicon semiconductor layer.Further, the “SOT substrate” includes in its category a structure inwhich a semiconductor layer is provided over an insulating substratesuch as a glass substrate with an insulating layer providedtherebetween. An example of the case where a single crystal siliconsubstrate is used as the substrate 10 is described herein.

A protective layer 102 serving as a mask for forming an elementisolation insulating layer is formed over the substrate 10 (see FIG.3A). As the protective layer 102, an insulating layer formed usingsilicon oxide, silicon nitride, silicon nitride oxide, or the like canbe used, for example. Before or after this step, an impurity elementimparting an n-type conductivity or an impurity element imparting ap-type conductivity may be added to the substrate 10 in order to controlthe threshold voltage of a semiconductor device. When the semiconductorof the substrate 10 is silicon, phosphorus, arsenic, or the like can beused as the impurity imparting the n-type conductivity, and boron,aluminum, gallium, or the like can be used as the impurity imparting thep-type conductivity.

Next, part of the substrate 10 in a region (an exposed region) which isnot covered with the protective layer 102 is etched using the protectivelayer 102 as a mask. Thus, an isolated semiconductor region 104 isformed (see FIG. 3B). As the etching, dry etching is preferablyemployed, but wet etching can alternatively be employed. An etching gasor an etchant thereof can be selected as appropriate depending on thematerial to be etched.

Next, an insulating layer is formed so as to cover the semiconductorregion 104 and is selectively removed in a region overlapping with thesemiconductor region 104, so that the element isolation insulatinglayers 106 are formed (see FIG. 3B). The insulating layer is formedusing silicon oxide, silicon nitride, silicon nitride oxide, or thelike. As a method for removing the insulating layer, polishing treatmentsuch as chemical mechanical polishing (CMP), etching treatment, or thelike can be given, and any of the treatments can be employed. Theprotective layer 102 is removed after formation of the semiconductorregion 104 or after formation of the element isolation insulating layers106.

Next, an insulating layer is formed over the semiconductor region 104,and a layer containing a conductive material is formed over theinsulating layer.

The insulating layer serves as a gate insulating film and preferably hasa single-layer structure or a stacked-layer structure using a filmcontaining silicon oxide, silicon nitride oxide, silicon nitride,hafnium oxide, aluminum oxide, tantalum oxide, or the like which isformed by a CVD method, a sputtering method, or the like. Alternatively,the insulating layer may be formed by oxidizing or nitriding a surfaceof the semiconductor region 104 by high-density plasma treatment orthermal oxidation treatment. The high-density plasma treatment can beperformed with, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and oxygen, nitrogen oxide, ammonia, nitrogen, or the like.The permittivity and thickness of the insulating layer are determineddepending on the channel length of the transistor; for example, thethickness of the insulating layer may be greater than or equal to 1 nmand less than or equal to 100 nm.

The layer containing a conductive material can be formed using a metalmaterial such as aluminum, copper, titanium, tantalum, or tungsten. Thelayer containing a conductive material may be formed using asemiconductor material such as polycrystalline silicon exhibiting highconductivity. There is no particular limitation on the method forforming the layer containing a conductive material; any film formationmethod such as an evaporation method, a CVD method, a sputtering method,or a spin coating method can be employed. An example of the case wherethe layer containing a conductive material is formed using a metalmaterial is described herein.

After that, the insulating layer and the layer containing a conductivematerial are selectively etched, so that the gate insulating film 108and the gate electrode 110 are formed (see FIG. 3C).

Next, an insulating layer 112 that covers the gate electrode 110 isformed (see FIG. 3C). Then, boron (B), phosphorus (P), arsenic (As), orthe like is added to the semiconductor region 104, so that the pair ofimpurity regions 114 a and 114 b with a shallow junction is formed (seeFIG. 3C). By formation of the pair of impurity regions 114 a and 114 b,the channel region 116 is formed in the semiconductor region 104 belowthe gate insulating film 108 (see FIG. 3C). The concentration of theimpurity added can be set as appropriate; the concentration ispreferably set to be high when the size of a semiconductor element isextremely decreased. Although the pair of impurity regions 114 a and 114b is formed after formation of the insulating layer 112 in this example,the insulating layer 112 may be formed after formation of the pair ofimpurity regions 114 a and 114 b.

Next, the sidewall insulating layer 118 is formed (see FIG. 3D). Aninsulating layer is formed so as to cover the insulating layer 112 andthen subjected to highly anisotropic etching, thereby forming thesidewall insulating layer 118 in a self-aligned manner. In this step,the insulating layer 112 may be partly etched to expose a top surface ofthe gate electrode 110 and part of top surfaces of the impurity regions114 a and 114 b.

Next, an insulating layer is formed to cover the gate electrode 110, thepair of impurity regions 114 a and 114 b, the sidewall insulating layer118, and the like. Then, boron (B), phosphorus (P), arsenic (As), or thelike is added to part of the impurity regions 114 a and 114 b, so thatthe pair of high concentration impurity regions 120 a and 120 b isformed (see FIG. 3E). If necessary, an impurity imparting the oppositeconductivity type may be added to the outer side of the highconcentration impurity region 120 a, 120 b to form a halo region. Afterthat, the insulating layer is removed, and a metal layer 122 is formedto cover the gate electrode 110, the sidewall insulating layer 118, thepair of high concentration impurity regions 120 a and 120 b, and thelike (see FIG. 3E). The metal layer 122 can be formed by any filmformation method such as a vacuum deposition method, a sputteringmethod, or a spin coating method. The metal layer 122 is preferablyformed using a metal material that reacts with the semiconductormaterial of the semiconductor region 104 to be a low-resistance metalcompound. Examples of such a metal material are titanium, tantalum,tungsten, nickel, cobalt, and platinum.

Next, heat treatment is performed thereon to react the metal layer 122with the semiconductor material, so that the pair of metal compoundregions 124 a and 124 b in contact with the pair of high concentrationimpurity regions 120 a and 120 b, respectively is formed (see FIG. 3F).When the gate electrode 110 is formed using polycrystalline silicon orthe like, a metal compound region is also formed in a region of the gateelectrode 110 in contact with the metal layer 122.

As the heat treatment, irradiation with a flash lamp can be employed,for example. Although it is needless to say that any another heattreatment may be employed, a method by which heat treatment for a timeas short as possible can be achieved is preferably used in order toimprove the controllability of chemical reaction in formation of themetal compound. The metal compound regions are formed by reaction of themetal material and the semiconductor material and have sufficiently highconductivity. With the metal compound regions, the electric resistancecan be sufficiently reduced and the element characteristics can beimproved. The metal layer 122 is removed after formation of the pair ofmetal compound regions 124 a and 124 b.

Next, the interlayer insulating layer 126 and the interlayer insulatinglayer 128 are formed thereon (see FIG. 3G). The interlayer insulatinglayers 126 and 128 can be formed using an inorganic insulating materialsuch as silicon oxide, silicon nitride oxide, silicon nitride, hafniumoxide, aluminum oxide, or tantalum oxide. The interlayer insulatinglayers 126 and 128 can also be formed using an organic insulatingmaterial such as polyimide or acrylic. A two-layer structure of theinterlayer insulating layer 126 and the interlayer insulating layer 128is employed in this example; however, the structure of an interlayerinsulating layer in one embodiment of the present invention is notlimited thereto. After formation of the interlayer insulating layer 128,a top surface of the interlayer insulating layer 128 is preferablyplanarized with CMP, etching treatment, or the like.

Then, openings that reach the pair of metal compound regions 124 a and124 b are formed in the interlayer insulating layers, and the sourceelectrode 130 a and the drain electrode 130 b are formed in the openings(see FIG. 3H). A conductive layer is formed by a PVD method, a CVDmethod, or the like in a region including the openings, and part of theconductive layer is removed by etching treatment, CMP, or the like,thereby forming the source electrode 130 a and the drain electrode 130b.

It is preferable that top surfaces of the source electrode 130 a and thedrain electrode 130 b be planar. For example, in the case where a thintitanium film or a thin titanium nitride film is formed in the regionincluding the openings and then a tungsten film is formed to be embeddedin the openings, part of tungsten, titanium, titanium nitride, or thelike can be removed as appropriately and the planarity of a top surfacethereof can be improved by CMP. By planarizing the top surface includingthe source electrode 130 a and the top surface of the drain electrode130 b in this manner, an electrode, a wiring, an insulating layer, asemiconductor layer, and the like can be formed in later steps moreappropriately.

Through the above, the transistor 160 using the substrate 10 containinga single crystal semiconductor material is formed.

<Structure Example of Wiring Layer 20 a, 20 b>

An example of a structure of the wiring layer 20 a, 20 b is describedbelow using FIG. 4.

The wiring layer 20 a shown in FIG. 4 includes an insulating layer 202,contact plugs 201 a and 201 b provided in openings of the insulatinglayer 202, an insulating layer 203 provided over the insulating layer202, and wirings 200 a and 200 b provided in openings of the insulatinglayer 203. The wiring layer 20 b has the same structure as the wiringlayer 20 a.

The insulating layer 202 is provided over the transistor 160 shown inFIG. 2. The contact plug 201 a is connected to the source electrode 130a of the transistor 160 and the wiring 200 a, and the contact plug 201 bis connected to the drain electrode 130 b of the transistor 160 and thewiring 200 b.

<Example of Manufacturing Method of Wiring Layer 20 a, 20 b>

Next, an example of a method for manufacturing the wiring layer 20 a, 20b is described using FIGS. 5A to 5H.

First, the insulating layer 202 is formed over the transistor 160 (seeFIG. 5A, though the transistor 160 is not shown). The insulating layer202 can have a single-layer structure or a stacked-layer structure usinga film containing an inorganic insulating material such as siliconoxide, silicon nitride oxide, or silicon nitride. For example, a stackedlayer of a silicon nitride film and a silicon oxide film can be used asthe insulating layer 202. In particular, in the case where the wirings200 a and 200 b contain copper, it is preferable to use a stacked-layerstructure in which a silicon nitride film with a thickness greater thanor equal to 5 nm and less than or equal to 50 nm is formed and a siliconoxide film with an appropriate thickness is stacked thereon in order toprevent dispersion of copper to the transistor 160. As a method forforming the insulating layer 202, a CVD method, a sputtering method, orthe like can be employed.

Next, a resist mask is formed over the insulating layer 202 by aphotolithography method or the like, and the insulating layer 202 isetched using the resist mask, so that openings 204 a and 204 b areformed (see FIG. 5B). In the case of employing a photolithographymethod, an antireflection film is preferably formed over the insulatinglayer 202 prior to the photolithography process; accordingly, reflectionof light on the conductive layer (e.g., the source electrode 130 a, thedrain electrode 130 b) or the like of the transistor 160 in the lightexposure by the photolithography method can be suppressed, i.e., areduction of the resolution in the photolithography method can besuppressed. A material of the antireflection film can be selected asappropriate depending on a material of the resist and the like. Althoughdry etching is preferably employed as the etching, wet etching canalternatively be employed. An etching gas or an etchant thereof can beselected as appropriate depending on the material to be etched.

Next, a layer 205 containing a conductive material is formed at least tofill the openings 204 a and 204 b (see FIG. 5C). As the layer 205, afilm containing a metal such as aluminum, titanium, tantalum, ortungsten, or a nitride or an alloy thereof, or the like can be used. Astacked-layer structure of the films can also be used as the layer 205.For example, a stacked layer of a titanium film, a titanium nitridefilm, and a tungsten film can be used as the layer 205. In particular,in the case where the wirings 200 a and 200 b contain copper, it ispreferable to use a titanium nitride layer with a thickness greater thanor equal to 5 nm and less than or equal to 50 nm as the layer 205 inorder to prevent dispersion of copper to the transistor 160. As a methodfor forming the layer 205, a CVD method, a sputtering method, or thelike can be employed.

Next, the layer 205 is removed at least to expose a top surface of theinsulating layer 202 by CMP (see FIG. 5D), so that the contact plugs 201a and 201 b are formed.

Next, the insulating layer 203 is formed over the insulating layer 202and the contact plugs 201 a and 201 b (see FIG. 5E). As the insulatinglayer 203, a single-layer structure or a stacked-layer structure using afilm containing an inorganic insulating material such as silicon oxide,silicon nitride oxide, or silicon nitride or a film using an insulatingmaterial such as silicone resin (so-called an SiOC film) formed fromorganosilane such as alkylsilane can be used. For example, a stackedlayer of an SiOC film and a silicon oxide film can be used as theinsulating layer 203. As a method for forming the insulating layer 203,a CVD method, a sputtering method, a spin coating method, or the likecan be employed.

Next, a resist mask is formed over the insulating layer 203 by aphotolithography method or the like, and at least the insulating layer203 is etched using the resist mask, so that grooves 206 a and 206 b areformed (see FIG. 5F). The grooves 206 a and 206 b pass through at leastthe insulating layer 203 to reach the contact plugs 201 a and 201 b,respectively. For example, the process time is controlled such that thegrooves 206 a and 206 b have an appropriate shape. Further, as describedabove, in the case of employing a photolithography method, anantireflection film is preferably formed over the insulating layer 203prior to the photolithography process. It is preferable to employ dryetching (particularly reactive ion etching) as the etching.

Next, a layer 207 containing a conductive material is formed at least tofill the grooves 206 a and 206 b (see FIG. 5G). As the layer 207, a filmcontaining a metal such as copper, aluminum, titanium, tantalum, ortungsten, or a nitride or an alloy thereof, or the like can be used. Astacked-layer structure of the films can also be used as the layer 207.For example, a stacked layer of a titanium nitride film and a copperfilm can be used as the layer 207. As a method for forming the layer207, a CVD method; a sputtering method; or a method in which a seedlayer is formed by a CVD method, a sputtering method, or the like andelectroplating is performed thereon; or the like can be employed.

As the layer 207, a wiring containing a film of copper or a copper alloyis preferably used; accordingly, the wiring resistance can be reduced.For example, the layer 207 can be formed in the following method: atantalum nitride layer with a thickness of from 5 nm to 50 nm is formedby a CVD method and a first copper layer with a thickness of from 5 nmto 50 nm is formed by a sputtering method or the like; and anelectroplating method is performed using the layers as an electrode, sothat a second copper layer is stacked. In that case, the tantalumnitride layer functions to prevent dispersion of copper downwardly andimprove the adhesion with the insulating layer 203, and the first copperlayer serves as a seed of the second copper layer.

Next, the layer 207 is removed at least to expose a top surface of theinsulating layer 203 by CMP (see FIG. 5H), so that the wirings 200 a and200 b are formed.

Through the above, the wiring layer 20 a is formed. The same process canbe applied to the wiring layer 20 b.

<Structure Example of Memory Cell 300>

An example of a structure of the memory cell 300 is described belowusing FIG. 6.

The memory cell 300 shown in FIG. 6 includes the transistor 301 and thecapacitor 302 one electrode of which is electrically connected to one ofa source and a drain of the transistor 301. Further, the transistor 301includes a pair of layers 3011 and 3013 containing conductive materials,which function as the source and the drain, a layer 3014 containing aconductive material, which functions as a gate, and an oxidesemiconductor layer 3012 which includes a channel region. The capacitor302 includes the layer 3013 which functions as one electrode and a layer3016 containing a conductive material, which functions as the otherelectrode. An insulating layer 3015 is provided between the layer 3014and the oxide semiconductor layer 3012, between the layer 3013 and thelayer 3016, and the like.

The layer 3011 is provided in an opening of an insulating layer 303. Theinsulating layer 303 is an insulating layer provided over the wiringlayer 20 b shown in FIG. 1, and the layer 3011 is connected to a wiring200 c in the wiring layer 20 b.

FIG. 6 also shows a memory cell 3000 which is next to the memory cell300 and a layer 3020 containing a conductive material, which functionsas a gate of a transistor included in a memory cell which is next to thememory cells 300 and 3000 in the direction perpendicular to the drawing.In FIG. 6, the layers 3014 and 3020 function as word lines in the memorycell array and the wiring 200 c functions as a bit line.

<Example of Manufacturing Method of Memory Cell 300>

Next, an example of a method for manufacturing the memory cell 300 isdescribed using FIGS. 7A to 7H.

First, the insulating layer 303 is formed over the wiring layer 20 b(see FIG. 7A, though the wiring layer 20 b is not shown). The insulatinglayer 303 can have a single-layer structure or a stacked-layer structureusing a film containing an inorganic insulating material such as siliconoxide, silicon nitride oxide, silicon nitride, hafnium oxide, aluminumoxide, or tantalum oxide or an organic insulating material such aspolyimide or acrylic. For example, a stacked layer of a silicon nitridefilm and a silicon oxide film can be used as the insulating layer 303.As a method for forming the insulating layer 303, a CVD method, asputtering method, a spin coating method, or the like can be employed.

Next, a resist mask is formed over the insulating layer 303 by aphotolithography method or the like, and the insulating layer 303 isetched using the resist mask, so that the openings are formed. In thecase of employing a photolithography method, an antireflection film ispreferably formed over the insulating layer 303 prior to thephotolithography process as described above. Although dry etching ispreferably employed as the etching, wet etching can alternatively beemployed. An etching gas thereof can be selected as appropriatedepending on the material to be etched.

Next, a layer 3001 containing a conductive material is formed at leastto fill the openings in the insulating layer 303 (see FIG. 7B). As thelayer 3001, a film containing a metal such as aluminum, titanium,tantalum, or tungsten, or a nitride or an alloy thereof, or the like canbe used. A stacked-layer structure of the films can be used as the layer3001. As a method for forming the layer 3001, a CVD method, a sputteringmethod, or the like can be employed.

Next, the layer 3001 is removed at least to expose a top surface of theinsulating layer 303 by CMP (see FIG. 7C), so that the layer 3011containing a conductive material which functions as a source or a drain,and the like are formed.

Next, an oxide semiconductor layer is formed over the insulating layer303 and the layer 3011. As an oxide semiconductor used for the oxidesemiconductor layer, the following metal oxide can be used: afour-component metal oxide such as an In—Sn—Ga—Zn—O based oxidesemiconductor; a three-component metal oxide such as an In—Ga—Zn—O basedoxide semiconductor, an In—Sn—Zn—O based oxide semiconductor, anIn—Al—Zn—O based oxide semiconductor, a Sn—Ga—Zn—O based oxidesemiconductor, an Al—Ga—Zn—O based oxide semiconductor, or a Sn—Al—Zn—Obased oxide semiconductor; a two-component metal oxide such as anIn—Zn—O based oxide semiconductor, a Sn—Zn—O based oxide semiconductor,an Al—Zn—O based oxide semiconductor, a Zn—Mg—O based oxidesemiconductor, a Sn—Mg—O based oxide semiconductor, an In—Mg—O basedoxide semiconductor, or an In—Ga—O based oxide semiconductor; anone-component metal oxide such as an In—O based oxide semiconductor, aSn—O based oxide semiconductor, or a Zn—O based oxide semiconductor; orthe like. In this specification, for example, the “In—Sn—Ga—Zn—O-basedoxide semiconductor” means a metal oxide containing indium (In), tin(Sn), gallium (Ga), and zinc (Zn), whose stoichiometric compositionratio is not particularly limited. The above oxide semiconductor maycontain silicon.

The oxide semiconductor layer is preferably formed by a method in whichimpurities such as hydrogen, water, a hydroxyl group, or hydride do notenter the oxide semiconductor layer as much as possible. For example,the oxide semiconductor layer can be formed by a sputtering method orthe like. A deposition atmosphere thereof may be a rare gas (typicallyargon) atmosphere, an oxygen atmosphere, or a mixed atmospherecontaining a rare gas and oxygen; an atmosphere of a gas purified bysufficiently removing an impurity such as hydrogen, water, a hydroxylgroup, or hydride is preferable, in order to prevent hydrogen, water, ahydroxyl group, hydride, or the like from entering the oxidesemiconductor layer.

Although the oxide semiconductor layer may be amorphous, a crystallineoxide semiconductor layer is preferably used for the channel region ofthe transistor. This is because the reliability (resistance to the gatebias stress) of the transistor can be improved.

Although the crystalline oxide semiconductor layer is ideally in asingle-crystal state, it is also preferable that the crystalline oxidesemiconductor layer include a crystal with c-axis alignment (alsoreferred to as c-axis aligned crystal (CAAC)). The crystal with c-axisalignment refers to a hexagonal crystal whose c-axis is perpendicular orsubstantially perpendicular to the surface where the crystal is provided(the top surface of the insulating layer 303 herein).

A sputtering method can be employed to form the oxide semiconductorlayer including CAAC. In order to obtain the oxide semiconductor layerincluding CAAC by a sputtering method, it is important to form ahexagonal crystal in an initial stage of deposition of the oxidesemiconductor layer and cause crystal growth from the hexagonal crystalas a seed; therefore, it is preferable that the distance between atarget and the substrate be set to be longer (e.g., about 150 mm toabout 200 mm) and the substrate heating temperature be 100° C. to 500°C., further preferably 200° C. to 400° C., still further preferably 250°C. to 300° C. In addition to that, the deposited oxide semiconductorlayer is preferably subjected to heat treatment at a temperature higherthan the substrate heating temperature, whereby microdefects in the filmand defects at the interface with a stacked layer can be repaired.

The oxide semiconductor layer including CAAC is highly purified, inwhich defects due to oxygen deficiency are reduced, and the oxidesemiconductor layer includes a c-axis crystal alignment, whereby valenceelectrons can be easily controlled to have low p-type conductivity.

Next, a resist mask is formed by a photolithography method or the like,and the oxide semiconductor layer is etched using the resist mask, sothat the oxide semiconductor layer 3012 and the like are formed (seeFIG. 7D). Dry etching is preferably employed as the etching. An etchinggas or an etchant thereof can be selected as appropriate depending onthe material to be etched.

After that, heat treatment may be performed on the oxide semiconductorlayer 3012. With the heat treatment, substances including hydrogen atomsin the oxide semiconductor layer 3012 can be further removed; thus, astructure of the oxide semiconductor layer 3012 can be ordered anddefect levels in the energy gap can be reduced. The heat treatment isperformed under an inert gas atmosphere at a temperature greater than orequal to 250° C. and less than or equal to 700° C., preferably greaterthan or equal to 450° C. and less than or equal to 600° C. or less thana strain point of the substrate. The inert gas atmosphere is preferablyan atmosphere which contains nitrogen or a rare gas (e.g., helium, neon,or argon) as its main component and does not contain water, hydrogen, orthe like. For example, the purity of nitrogen or a rare gas such ashelium, neon, or argon introduced into a heat treatment apparatus is setto be greater than or equal to 6 N (99.9999%), preferably greater thanor equal to 7 N (99.99999%) (that is, the concentration of impurities isless than or equal to 1 ppm, preferably less than or equal to 0.1 ppm).

The impurities can be reduced by the heat treatment, leading to ani-type oxide semiconductor film (an intrinsic oxide semiconductor film)or a substantially i-type oxide semiconductor film, which enables atransistor having extremely high characteristics to be formed.

Next, an insulating layer 3002 is formed over the insulating layer 303and the oxide semiconductor layer 3012 (see FIG. 7E). The insulatinglayer 3002 functions as a gate insulating film of the transistor 301.The insulating layer 3002 can have a single-layer structure or astacked-layer structure using a film containing an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, hafnium oxide,aluminum oxide, or tantalum oxide. As a method for forming theinsulating layer 3002, a sputtering method or the like can be employed.

Next, a layer containing a conductive material is formed over theinsulating layer 3002. As the layer, a film containing a metal such asaluminum, titanium, tantalum, or tungsten, or a nitride or an alloythereof, or the like can be used. An oxide such as indium oxide,tungsten oxide, or molybdenum oxide, or a nitride such as indium nitrideor zinc nitride can also be used. Further, a stacked-layer structure ofthe films can also be used as the layer. As a method for forming thelayer, a CVD method, a sputtering method, or the like can be employed.

Next, a resist mask is formed over the layer containing a conductivematerial by a photolithography method or the like, and the layer isetched using the resist mask, so that the layers 3014 and 3020containing conductive materials and the like are formed (see FIG. 7F).Although dry etching is preferably employed as the etching, wet etchingcan alternatively be employed. An etching gas or an etchant thereof canbe selected as appropriate depending on the material to be etched.

Next, an insulating layer 3003 is formed over the insulating layer 3002and the layers 3014 and 3020. The insulating layer 3003 can have asingle-layer structure or a stacked-layer structure using a filmcontaining an inorganic insulating material such as silicon oxide,silicon nitride oxide, or silicon nitride or an organic insulatingmaterial such as polyimide or acrylic. As a method for forming theinsulating layer 3003, a CVD method, a sputtering method, a spin coatingmethod, or the like can be employed.

Next, a resist mask is formed over the insulating layer 3003 by aphotolithography method or the like, and the insulating layer 3003 isetched using the resist mask, so that openings are formed. Although dryetching is preferably employed as the etching, wet etching canalternatively be employed. An etching gas or an etchant thereof can beselected as appropriate depending on the material to be etched.

Next, a layer 3004 containing a conductive material is formed at leastto fill the openings in the insulating layer 3003 (see FIG. 7G). As thelayer 3004, a film containing a metal such as aluminum, titanium,tantalum, or tungsten, or a nitride or an alloy thereof, or the like canbe used. A stacked-layer structure of the films can also be used as thelayer 3004. As a method for forming the layer 3004, a CVD method, asputtering method, or the like can be employed.

Next, the layer 3004 is removed at least to expose a top surface of theinsulating layer 3003 by CMP (see FIG. 7H), so that a layer 3005containing a conductive material which functions as a source or a drainof the memory cell 300, and the like are formed.

Through the above, the transistor 301 included in the memory cell 300 isformed. The capacitor (stack capacitor) 302 in the memory cell 300 canbe formed as appropriate by a known method.

<Semiconductor Memory Device Disclosed in this Specification>

A semiconductor memory device disclosed in this specification includes adriver circuit including part of a substrate containing a single crystalsemiconductor material, a multilayer wiring layer provided over thedriver circuit, and a memory cell array layer provided over themultilayer wiring layer. That is, the driver circuit overlaps with thememory cell array in the semiconductor memory device disclosed in thisspecification. Accordingly, the integration degree of the semiconductormemory device can be increased as compared to the case where a drivercircuit and a memory cell array are provided in the same plane using asubstrate containing a single crystal semiconductor material.

As a wiring contained in the multilayer wiring layer, a wiring of copperor a copper alloy is preferably used; accordingly, the wiring resistanceof the wiring can be reduced, i.e., an operation delay of thesemiconductor memory device can be suppressed. This effect isparticularly large in the case where a wiring of copper or a copperalloy is used as a wiring (so-called a bit line) used for data writingand data reading to/from a memory cell.

Further, a transistor whose channel region is formed of an oxidesemiconductor is preferably used as a transistor included in the memorycell. This is because the off-state current of a transistor whosechannel region is formed of a semiconductor having a wide band gap, suchas an oxide semiconductor, is extremely small as compared to theoff-state current of a transistor using another semiconductor such assilicon. Accordingly, the leakage of electrical charge from a capacitorcan be suppressed in the memory cell in the semiconductor memory devicedisclosed in this specification. Consequently, the frequency of arefresh operation can be reduced. In this manner, power consumption canbe reduced by reducing the frequency of a refresh operation in thesemiconductor memory device disclosed in this specification.

Further, a stack capacitor is preferably used as the capacitor in thememory cell; accordingly, both of high capacitance and high integrationof the memory cell can be achieved. Moreover, the semiconductor memorydevice disclosed in this specification is preferable in the followingpoint as compared to a conventional semiconductor memory device in whicheach memory cell includes a stack capacitor or a trench capacitor. Theconventional semiconductor memory device refers to a semiconductormemory device in which a transistor included in the memory cell isprovided using a substrate containing a single crystal semiconductormaterial and a multilayer wiring layer is provided over the memory cell.

The semiconductor memory device disclosed in this specification ispreferable in that a bit line is next to neither a pair of electrodes ofthe stack capacitor nor a word line. This is because in the memory cellarray of the semiconductor memory device disclosed in thisspecification, the word line (e.g., the layer 3014, 3020 containing aconductive material) and the pair of electrodes (e.g., the layers 3013and 3016 containing conductive materials) of the capacitor are providedon a side which is opposite to the bit line (the wiring 200 c) withrespect to the transistor 301, whereas in the conventional semiconductormemory device, a bit line and at least one of a pair of electrodes of acapacitor and a word line are provided on the same side of thetransistor in the memory cell. Accordingly, in the semiconductor memorydevice disclosed in this specification, power consumption can be reducedand an operation delay can be suppressed by reducing the parasiticcapacitance of each wiring (particularly the bit line), for example.

In addition, the structure where the capacitor 302 and the wiring 200 care provided with the transistor 301 provided therebetween leads to anincrease in the freedom of design of the capacitor 302 and the wiring200 c, which enables a capacitor with an appropriate capacitance to beformed in an area as small as possible.

<Application Example of Semiconductor Memory Device>

An application example of the above-described semiconductor memorydevice is described below using FIG. 8.

FIG. 8 is a block diagram showing a structure example of amicroprocessor. The microprocessor illustrated in FIG. 8 includes a CPU401, a main memory 402, a clock controller 403, a cache controller 404,a serial interface 405, an I/O port 406, terminals 407, an interface408, a cache memory 409, and the like. It is needless to say that themicroprocessor illustrated in FIG. 8 is just an example of thesimplified structure, and practical microprocessors have variousstructures depending on their usages.

In order to operate the CPU 401 at high speed, a high-speed memorymatched for the speed of the CPU 401 is needed. However, a high-speedlarge capacity memory whose access time is matched for the operationspeed of the CPU 401 generally costs high. Thus, in addition to the mainmemory 402 having large capacity, the cache memory 409 which is ahigh-speed memory having smaller capacity than the main memory 402, suchas an SRAM, is provided between the CPU 401 and the main memory 402. TheCPU 401 accesses the cache memory 409, thereby operating at high speedregardless of the speed of the main memory 402.

In the microprocessor illustrated in FIG. 8, the above-describedsemiconductor memory device can be used for the main memory 402.According to the above structure, a highly integrated, highly reliablemicroprocessor can be provided.

A program to be executed in the CPU 401 is stored in the main memory402. The program stored in the main memory 402 is downloaded to thecache memory 409 in the initial execution, for example. Not only theprogram stored in the main memory 402 but also a program in any externalmemory can be downloaded. The cache memory 409 not only stores theprogram executed in the CPU but also functions as a work region andtemporarily stores the calculation results or the like of the CPU 401.

The number of CPUs is not limited to one; a plurality of CPUs may beprovided. By processing in parallel with a plurality of CPUs, theoperation speed can be improved. In that case, if the processing speedsof the CPUs are uneven, malfunction may occur in some cases as a wholeprocessing; hence, the processing speed of each CPU which is a slave maybe balanced by the rest of the CPUs which is/are a master/masters.

Although the microprocessor is given as an example herein, the usage ofthe above-described semiconductor memory device is not limited to themain memory of the microprocessor. For example, the above-describedsemiconductor memory device is also preferably used as a video RAM whichis used in a driver circuit of a display device or a large capacitymemory which is involved in an image processing circuit. Besides, alsoin a variety of system LSIs, the above-described semiconductor memorydevice can be used as a large capacity memory or a small-sized memory.

Example 1

Examples of a semiconductor device having the above-describedsemiconductor memory device are described in this example. Thesemiconductor memory device according to one embodiment of the presentinvention leads to a reduction in the size of the semiconductor device.In particular, in the case of a portable semiconductor device, anadvantage in improving convenience of users can be provided through thedownsizing with the semiconductor memory device according to oneembodiment of the present invention.

The semiconductor memory device according to one embodiment of thepresent invention can be used for display devices, laptops, or imagereproducing devices provided with recording media (typically, deviceswhich reproduce the content of recording media such as digital versatilediscs (DVDs) and have displays for displaying the reproduced images).Other than the above, as examples of the semiconductor device to whichthe semiconductor memory device according to one embodiment of thepresent invention can be applied, mobile phones, portable game machines,portable information terminals, e-book readers, cameras such as videocameras or digital still cameras, goggle-type displays (head mounteddisplays), navigation systems, audio reproducing devices (e.g., caraudio systems and digital audio players), copiers, facsimiles, printers,multifunction printers, automated teller machines (ATM), vendingmachines, and the like can be given. FIGS. 9A to 9C illustrate concreteexamples of the semiconductor devices.

FIG. 9A illustrates a portable game machine including a housing 7031, ahousing 7032, a display portion 7033, a display portion 7034, amicrophone 7035, speakers 7036, an operation key 7037, a stylus 7038,and the like. The semiconductor memory device according to oneembodiment of the present invention can be applied to an integratedcircuit for controlling driving of the portable game machine. With theuse of the semiconductor memory device according to one embodiment ofthe present invention for the integrated circuit for controlling drivingof the portable game machine, a compact portable game machine can beprovided. Although the portable game machine illustrated in FIG. 9A hastwo display portions, 7033 and 7034, the number of display portionsincluded in the portable game machine is not limited to two.

FIG. 9B illustrates a mobile phone including a housing 7041, a displayportion 7042, an audio-input portion 7043, an audio-output portion 7044,operation keys 7045, a light-receiving portion 7046, and the like. Lightreceived in the light-receiving portion 7046 is converted intoelectrical signals, whereby external images can be loaded. Thesemiconductor memory device according to one embodiment of the presentinvention can be applied to an integrated circuit for controllingdriving of the mobile phone. With the use of the semiconductor memorydevice according to one embodiment of the present invention for theintegrated circuit for controlling driving of the mobile phone, acompact mobile phone can be provided.

FIG. 9C illustrates a portable information terminal including a housing7051, a display portion 7052, operation keys 7053, and the like. In theportable information terminal illustrated in FIG. 9C, a modem may beincorporated in the housing 7051. The semiconductor memory deviceaccording to one embodiment of the present invention can be applied toan integrated circuit for controlling driving of the portableinformation terminal. With the use of the semiconductor memory deviceaccording to one embodiment of the present invention for the integratedcircuit for controlling driving of the portable information terminal, acompact portable information terminal can be provided.

This application is based on Japanese Patent Application serial no.2011-005401 filed with Japan Patent Office on Jan. 14, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a firstinsulating layer; a second insulating layer over the first insulatinglayer; a contact plug provided in an opening of the first insulatinglayer; and a wiring provided in an opening of the second insulatinglayer, wherein the wiring is within an opening of the first insulatinglayer and is in contact with a side surface of the contact plug.
 2. Asemiconductor device comprising: a first insulating layer; a secondinsulating layer over the first insulating layer; a first contact plugand a second contact plug provided in openings of the first insulatinglayer; and a first wiring and a second wiring provided in openings ofthe second insulating layer, wherein the first wiring is within anopening of the first insulating layer and is in contact with a sidesurface of the first contact plug.
 3. A semiconductor device comprising:a first insulating layer; a second insulating layer in contact with thefirst insulating layer; a first layer containing a conductive materialprovided in an opening of the first insulating layer; and a second layercontaining a conductive material provided in an opening of the secondinsulating layer, wherein the second layer is within an opening of thefirst insulating layer and is in contact with a side surface of thefirst layer.
 4. The semiconductor device according to claim 1, furthercomprising: a third insulating layer in contact with the firstinsulating layer; and a transistor, the transistor comprising: a sourceelectrode or a drain electrode electrically connected to the contactplug; and a gate electrode overlapping with the first insulating layerand the second insulating, wherein the contact plug comprises aconductive material, wherein the source electrode or the drain electrodeextend in a same direction as the contact plug, and wherein the gateelectrode does not overlap with the contact plug and the wiring.
 5. Thesemiconductor device according to claim 2, further comprising: a thirdinsulating layer in contact with the first insulating layer; and atransistor, the transistor comprising: a source electrode electricallyconnected to the first contact plug; a drain electrode electricallyconnected to the second contact plug; and a gate electrode, wherein thesource electrode and the drain electrode are in contact with a bottomsurface of the first contact plug and the second contact plug,respectively, wherein the bottom surface of the first contact plug issmaller than a top surface of the source electrode, wherein the gateelectrode is provided between the first wiring and the second wiring,and wherein the first contact plug and the second contact plug comprisea conductive material.
 6. The semiconductor device according to claim 4,wherein the wiring comprises copper or a copper alloy.
 7. Thesemiconductor device according to claim 5, wherein the first wiring andthe second wiring comprise copper or a copper alloy.
 8. Thesemiconductor device according to claim 2, wherein the first wiring andthe second wiring comprise copper or a copper alloy.
 9. Thesemiconductor device according to claim 1, wherein a bottom surface ofthe wiring is below a top surface of the first insulating layer.
 10. Thesemiconductor device according to claim 2, wherein a bottom surface ofthe first wiring and a bottom surface of the second wiring are below atop surface of the first insulating layer.
 11. The semiconductor deviceaccording to claim 3, further comprising: a third insulating layer incontact with the first insulating layer; and a transistor, thetransistor comprising: a source electrode electrically connected to thefirst layer; a drain electrode electrically connected to a third layerprovided in the first insulating layer; and a gate electrode, whereinthe source electrode and the drain electrode are in contact with abottom surface of the first layer and the third layer, respectively,wherein the bottom surface of the first layer is smaller than a topsurface of the source electrode, wherein the gate electrode is providedbetween the second layer and a fourth layer provided in the secondinsulating layer, and wherein a bottom surface of the second layer isbelow a top surface of the first insulating layer.
 12. The semiconductordevice according to claim 1, wherein a top surface of the contact plugis convex.
 13. The semiconductor device according to claim 2, wherein atop surface of the first contact plug is convex.
 14. The semiconductordevice according to claim 3, wherein a top surface of the first layer isconvex.
 15. The semiconductor device according to claim 1, wherein thewiring is electrically connected to a source or drain of a transistor.16. The semiconductor device according to claim 2, wherein the firstwiring is electrically connected to a source or drain of a transistor.17. The semiconductor device according to claim 3, wherein the secondlayer is electrically connected to a source or drain of a transistor.18. The semiconductor device according to claim 1, wherein the firstinsulating layer, the second insulating layer, the contact plug, and thewiring belong to a first wiring layer, wherein the semiconductor devicefurther comprises: a second wiring layer over the first wiring layer; adriver circuit provided below the first wiring layer; and a plurality ofmemory cells provided over the second wiring layer, wherein a transistorprovided in the driver circuit is electrically connected to one of theplurality of memory cells via the first and second wiring layers. 19.The semiconductor device according to claim 2, wherein the firstinsulating layer, the second insulating layer, the first contact plug,the second contact plug, the first wiring, and the second wiring belongto a first wiring layer, wherein the semiconductor device furthercomprises: a second wiring layer over the first wiring layer; a drivercircuit provided below the first wiring layer; and a plurality of memorycells provided over the second wiring layer, wherein a transistorprovided in the driver circuit is electrically connected to one of theplurality of memory cells via the first and second wiring layers. 20.The semiconductor device according to claim 3, wherein the firstinsulating layer, the second insulating layer, the first layer, and thesecond layer belong to a first wiring layer, wherein the semiconductordevice further comprises: a second wiring layer over the first wiringlayer; a driver circuit provided below the first wiring layer; and aplurality of memory cells provided over the second wiring layer, whereina transistor provided in the driver circuit is electrically connected toone of the plurality of memory cells via the first and second wiringlayers.